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doi:10.2204/iodp.proc.332.104.2011

Discussion

In this section, we discuss two aspects of Hole C0002G drilling and observatory results. First, we review the LWD/MWD results of Expedition 332 and compare them to the records of Hole C0002A drilled in 2007 during Expedition 314 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Second, we examine the LTBMS installation procedure in the context of the difficult environmental conditions of the strong Kuroshio Current.

LWD

As described in “Downhole measurements,” the unit boundaries identified during Expedition 332 are a good match for those identified in Hole C0002A during Expedition 314 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). The first boundary dividing the Quaternary forearc unit (Unit I) and the underlying lower forearc unit (Unit II) is represented in both LWD and MWD data sets as a sudden, constant trend to higher resistivity values. The transition between the lower forearc unit and the basal forearc unit (Unit III) shows an equally significant decrease in amplitude and frequency in the resistivity log. The lowermost boundary between the basal forearc unit and the accretionary prism (Unit IV) shows the opposite behavior: both gamma ray and resistivity experience a sudden drop, followed by a highly oscillating signal in the accreted mudstones. The absolute differences in depth between the LWD/MWD measurements from Holes C0002A and C0002G are minor, ranging between 3 and 6 m. Despite deformation and faulting in the upper prism and a complex depositional history of the Kumano Basin being filled by turbiditic sequences, no major material changes or depth variations of the boundaries occur (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009).

Long-term borehole observatory installation

Given the potential hazard of unfavorable current velocities and resulting VIV along the drill string, precautions were taken to minimize VIV by attaching ropes to the upper portion of the drill string above the CORK head assembly (see Fig. F13 in the “Expedition 332 summary” chapter [Expedition 332 Scientists, 2011a]). This operational measure turned out to be very successful for Site C0002 and kept VIV forces to a minimum, which was confirmed by ROV surveys and observation in the moonpool area as well as accelerometer recordings on the drill string.

In addition, special precautions were taken to secure the subseafloor instrument string and its various components. At short notice, centralizers were ordered and supplied that facilitated the installation of stainless steel bands or, alternatively, plastic tie wraps (see Fig. F26 in the “Methods” chapter [Expedition 332 Scientists, 2011b]). The spacing of the centralizers was ~2 m below the swellable packer (four units per joint of tubing) and ~4–5 m above the packer (two units per joint). Between the centralizers, numerous tie wraps and metal bands were carefully attached to hold the three cables, the thermistor string (where present), and the flatpack containing the hydraulic lines in place. Where needed, additional duct tape was attached to avoid slack in the lines running up. Spacing of the bands and wraps could be as close as every 20 cm in places, particularly in areas where the completion assembly outside diameter increased (i.e., near the instrument carrier, centralizers, packer assembly, etc.). The procedure turned out to be time-consuming but highly successful, and ROV surveys in the water column confirmed that everything remained in place during the entire deployment. VIV was also kept to a minimum during drifting and landing (see “Vortex-induced vibration measurements”), and the good health of all observatory components was confirmed after having completed Hole C0002G.

ROV operations further helped constrain the various design modifications on the LTBMS compared to earlier CORKs (e.g., Becker and Davis, 2005; Mikada, Becker, Moore, Klaus, et al., 2002). It turned out to be an advantage to have the ODI connector of the pressure unit in a horizontal position, with the sensitive portion of the instrument below shielded by a stainless steel cover. This facilitates and hence shortens ROV bottom time because the pilots can fly in with a stretched-out manipulator arm without being overly cautious of damaging something in case they slightly miss the connector on occasion. The vertically aligned ODI connectors proved to be less practical because they cannot be reached by the ROV as effortlessly and also impose the risk of damage because shear forces are applied if the vehicle fails to push or pull in a perfectly vertical direction.

The communication lines can be seen as a major achievement in this LTBMS development in that checks of the instrument’s functionality could be carried out at (almost) all times. Given the special adapter and ROV interface unit, plus an additional onboard interface board (Fig. F21), testing was possible with the programmed applications and a portable mini-PC. These tests were first carried out on board the ship in a containerized lab shack and were then transferred to the ROV shack once the CORK head was launched into the water (see details in “Long-term borehole monitoring system”). Such operations are possible on the Chikyu but are also desirable for future installations (e.g., on the R/V JOIDES Resolution) because they help minimize the risk of instrument loss or malfunction and optimize operations.

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